The invention relates to a scanning head of the measuring type for a coordinate measuring device. These scanning heads consist of a fixed part that is connected with a device that can freely move and position the scanning head in three spatial dimensions, within a defined measurement volume, usually a so-called coordinate measurement device, as well as of three parts that are movable with regard to the fixed part. The movable parts are frequently configured as rockers with a parallel spring plate guide, which allows precise guidance in one spatial direction. The movement directions are usually disposed orthogonal to one another, and all of the other degrees of freedom are set. The movement directions of the three rockers build on one another kinematically, so that the last rocker of the kinematic chain is freely movable within a movement play space in the form of a cube having an edge length of typically a few millimeters. Furthermore, re-set elements are provided, which bring the scanning head rockers into a resting position, which lies approximately in the center of the movement play space, if no external forces are acting on the rockers. Furthermore, position measurement devices are provided, which measure the deviation from the resting position of the individual rockers, and thereby define a deflection vector of the kinematically last rocker. The zero points of these position measurement devices usually define the said resting position.
Usually, a mechanical interface is provided on the last rocker, which allows an automatic switching of so-called feeler combinations. This interface consists of a kinematically defined bearing that precisely establishes all six kinematic degrees of freedom, so that during repeated setting of the same feeler combinations, the relative position between the scanning head rocker and this feeler combination is reproduced as precisely as possible.
In the case of tactile measurement of work pieces, using a coordinate measurement device, to which a scanning head as described above is attached, undesirable vibrations both of the support structures of the coordinate measurement device and of the scanning head rockers occur, particularly during continuous scanning of work pieces, leading to measurement errors. Vibrations of the scanning head rockers, in particular, result in oscillations of the scanning forces, which in turn result in changes in the feeler bending, which are not detected during feeler calibration and therefore cannot be corrected. This becomes evident, in particular, in the case of long and easily bendable feelers. There are a number of proposals in the state of the art with regard to this problem, as to how these undesirable vibrations can be damped.
U.S. Pat. No. 5,088,208 includes a measuring scanning head that carries a damping device that consists of a cylinder in which a viscous fluid (silicone) is located, and a punch that projects into this fluid. The damping effect is achieved in that the punch and the cylinder are rigidly connected with the components of the scanning head that move relative to one another, in each instance.
This damping device, which is part of the state of the art, has the disadvantage that the fluid can flow out of the cylinder. This problem was solved in U.S. Pat. No. 5,088,208 by means of an elastomer cuff that surrounds the punch and the cylinder. This solution, however, in turn has the disadvantage that the cuff exerts undefined additional forces on the parts of the scanning head that move towards one another. Furthermore, such a cuff cannot be configured to be absolutely leak-proof, and this can result in undesirable creep of the silicone out of the cuff. Furthermore, this passive damping device has the disadvantage that it is always engaged, and this results in additional forces that in turn bend the scanning pin in case of fast changes in load at the scanning head rocker, and this results in measurement errors.
If a friction-free or at least low-friction electric motor, for example a plunger-type coil system or a different type of DC linear motor is used, which is connected with the mechanical vibration system, speed-proportional damping that is adjustable to almost any desired value can be set. This electric motor is disposed between the movable part and the fixed part of a rocker, in such a way that it can exert forces on the movable part. Of course, motors that act in rotary manner can also be used, which are mechanically coupled with the rockers in such a manner that the rotation movement is converted to a linear movement. According to the state of the art, such motors are controlled by feedback control circuits in order to damp vibrations; these control circuits use at least one status variable of the mechanical vibration system in order to generate a braking force that acts opposite to the movement direction of the mechanical vibration system. This is done in that the regulator supplies the electric motor with electrical power, as a function of the observed status variable, so that the desired braking effect is achieved. However, stability criteria of regulation technology must be adhered to. If tactile scanning is used as the basis, greatly varying feeler weights, and, depending on the scanning direction, greatly different spring stiffness values of the bending feelers, result in a large parameter bandwidth that generally results in a compromise on the basis of the stability criteria that must be met (for example, great damping is given up), or results in a complicated regulation strategy.
German Patent No. DD 150 111 includes a device for path/force coordination on feeler systems for scanning heads on coordinate measurement devices. This reference describes how damping of the movement of the movable part of a scanning head for coordinate measurement systems is performed. For this purpose, the path signal of the path measurement device of the movable scanning head part, in each instance, is applied as an override to the linear motor, by means of a differential regulator (D regulator) of the path measurement device. The D regulator differentiates the path signal and thereby obtains a signal proportional to the speed, which is processed further by the regulator. The disadvantages of this regulator have already been described above. The advantages of the regulators lie in the flexibility, since the amount of the damping force can be adjusted by means of changing the regulator parameters.
Furthermore, a scanning head for coordinate measurement devices having a damping device that is based on the eddy current principle is described in European Patent No. 0 693 669 B1. For this purpose, it is necessary to introduce an additional piece of metallic conductor in the field region of the magnets of the linear motor described in EP 0 693 669 B1 (plunger-type coils), which piece is displaced relative to the magnetic field lines when the movable part of the scanning head is deflected. According to EP 0 693 669 B1, the use of metallic coil bodies is proposed for the plunger-type coils. By means of the displacement, eddy currents that are closed in themselves are induced in the metallic coil body, which currents in turn establish a magnetic field that is directed opposite to the magnetic field that generates them. In this way, a force is generated that is directed opposite to the movement and thereby damps it. It is a disadvantage that the amount of the current and therefore of the counter-force is limited by the electrical resistance that is inherent to every electrical conductor at room temperature. Furthermore, the proportionality factor, which links the speed of the movement to be damped with the damping force, cannot be changed. The damping force is proportional to the speed of the movable scanning head part and is dependent, in terms of its amount, only on the magnetic field intensity, the geometry of the components, and the specific resistance of the metal part in which the eddy currents flow. The advantage of this damping device lies in its sturdiness, since in contrast to the regulator, no stability criteria need to be observed.